1. Field of the Invention
The present invention relates to an optical pick-up actuator, and more particularly, to an optical pick-up actuator that can minimize vibration generated by a yoke plate.
2. Description of the Related Art
Generally, in a device for reading and writing information using an optical disk, an optical pick-up actuator functions to allow a laser beam transmitted through an object lens to be accurately located on a track formed on a surface of the disk. With the storage capacity of the optical disk being increased, it is increasingly required to provide a more precision driving control of the optical pick-up actuator.
In order to provide a more precision driving control, the number of apertures of the object lens is increased. However, the increase in the number of apertures inevitably generates aberration due to the tilt of the disk. The tilt of the disk deteriorates the playback function and makes it difficult to form pits in the course of writing information.
Therefore, there is a need for a tilt motion device for correcting the tilt of the disk while performing the tracking operation. There are two typical methods of correcting the tilt: a method of correcting the tilt by moving the overall body of an actuator using a DC motor, and a method of correcting the tilt by moving only an optical pick-up motion part (a lens holder) of the actuator.
The former method has a problem that it corrects only a disk in a low frequency band and increases the overall size of the optical disk player.
In order to perform the latter method, two types of optical pick-up actuators, a moving coil type and a moving magnet type, have been proposed. The moving coil type has at least six wires connected to a driving side to control the tilt motion, thereby complicating the structure of the actuator. The moving magnet type has a disadvantage of making it difficult to obtain enough sensitivity required for the object lens of the lens holder.
FIGS. 1 through 4 show a motion type optical pick-up actuator according to a related art. Referring to FIGS. 1 through 4, the optical pick-up actuator of the related art includes an object lens 11 for focusing a laser beam to/from an optical disk, a tracking coil 12 and a focusing coil 13 both installed on a rear side of the object lens 11, a lens holder 10 provided at a rear surface with tilt magnets 15, and front and rear yokes 22 for fixing magnets 21 to realize tracking and focusing motions of the tracking and focusing coils 12 and 13.
The optical pick-up actuator further includes a yoke plate 20 spaced apart from the front and rear yokes 22 and provided with a tilt yoke 23, a plurality of wire suspensions 30 disposed on both sides of the lens holder 10 and electrically connected to the tracking and focusing coils 12 and 13, a frame 40 provided at a rear surface with a board 43 for electrically connecting the wire suspensions 30 to each other, and a tilt coil 50 disposed around a yoke insertion member 42 to generate electromagnetic force around the tilt magnets 15.
The frame 40 is provided with fixing parts 41, and the yoke insertion member 42 for receiving the tilt yoke 23 is disposed between the fixing parts 41. The wire suspensions 30 extend through the fixing parts 41 and are connected to each other by the board 43 attached on the rear surface of the frame 40.
The tilt magnets 15 having opposite polarities, are disposed opposing the tilt coil 50 and the lens holder 10. The lens holder 10 is provided at a rear surface with fitting grooves 16 for receiving the tilt magnets 15. The fitting grooves 16 are formed on the left and right sides based on a central line of the lens holder 10 on which the object lens 11 is installed. Instead of forming the fitting grooves 16, adhesive may be used to attach the tilt magnets 15 on the rear surface of the lens holder 10. Alternatively, the tilt magnets 15 may be further provided on a front surface of the lens holder 10. In this case, plural tilt magnets 15 correspond to a single coil 50 in a state where the tilt yoke 23 is formed on a yoke plate 20.
The operation of the above described optical pick-up actuator of FIG. 1 will be described hereinafter.
The focusing and tracking motions of the optical pick-up actuator are realized by electromagnetic force generated by the magnets 21 attached on the yokes 22 and the focusing and tracking coils 13 and 12 of the lens holder 10, thereby reading and writing data.
The tilt motion of the actuator for eliminating the tilt aberration caused by the high RPM of the optical disk is realized by electromagnetic force generated by the tilt magnets 15 installed on the lens holder 10 and the tilt coils 50 installed on the tilt yoke 23 to tilt the lens holder 10. That is, the tilt magnets 15 installed on the left and right sides of the lens holder 10 holding the object lens 11 are, as shown in FIG. 4, disposed to have an polarity opposite to each other, whereby the tilt motion is carried out using the electromagnetic force generated by the tilt coil 50 disposed facing the tilt magnets 15.
In FIG. 4, the reference characters B, i and F indicate respectively electromagnetic field, current, and Lorentz force. The Lorentz force F is generated by interaction of the electromagnetic field B and the current i. The arrows beside the characters in FIG. 4 indicate the directions of the electromagnetic field, the current, and the Lorentz force. That is, since the current i flows in a predetermined direction and the tilt magnets 15 have polarity opposite to each other, coupled force is applied to the lens holder 10 on which the object lens 11 is installed. Since the coupled force corresponds to a moment applied in the direction of an X-axis, the lens holder 10 tilts in a seesaw motion about the X-axis.
As the tilt magnets 15 and the tilt coil 50 are disposed spacing away from the focusing and tracking coils 13 and 12, such a tilt motion can be realized independent from the focusing and tracking motions.
By maintaining a predetermined gap dx between the tilt magnets 15 and the tilt coil 50, magnetic flux between the tilt magnets 15 and the coil 50 is not varied in the course of performing the tracking motion.
Therefore, when compared with a related art hybrid-type optical pick-up actuator, the related art 3-axis motion type optical pick-up actuator has an advantage of increasing an output constant in the direction of the tilting motion as the gap dx between the tilt magnets 15 and the tilt coil 50 is set to have a minimal distance when the tilt motion is realized, without affecting the tracking and focusing motions in a state where the electromagnetic force is generated by the tilt magnets 15 and the coil 50.
As described above, the above-described 3-axis motion type optical pick-up actuator performs the 3-axis motion (focusing, tracking, and tilting motions) by providing the plural tilt magnets 15 formed on the lens holder 10 and the tilt coil 50 independently installed in the yoke insertion member 42 between the fixing parts 41 of the frame 40.
However, the tilt magnets 15 separately installed on a rear surface of the lens holder 10 cause the size and weight of the lens holder to be increased, which deteriorates the productivity and sensitivity of the pickup actuator. Furthermore, additional insertion and supporting structures for a magnetic circuit performing a radial tilt must be provided. In addition, since the wire suspension is composed of a 4-wire spring and a 2-coil spring to perform the 3-axis motion, this complicates the structure of the pickup actuator and deteriorates the productivity of the pickup actuator. Since the axis of the radial tilt is located on a rear side of the lens, an offset in the direction of the Z-axis is incurred. Since the front and rear yokes and the tilt yoke are bent in an L-shape from the yoke plate to receive the magnets for performing the 3-axis motion, the vibration generated by the front and rear yokes and the tilt yoke are directly transmitted to the yoke plate, which deteriorates the pick-up property. Furthermore, since the yokes are integrally bent from the yoke plate at right angles, it is difficult to process a mold and to couple the frame to the yoke plate.
FIGS. 5A through 7B show another structure of a yoke plate and frame assembly of an optical pickup actuator according to a related art.
A yoke vibration characteristic of the related art yoke plate will be described hereinafter with reference to FIGS. 5A through 7B.
As shown in FIGS. 5A and 5B illustrating respectively the top and bottom views of a yoke plate 70, the yoke plate 70 includes a first inner yoke 71 and a plurality of second inner yokes 72 all integrally extending from a bottom 76 of the yoke plate 70. Magnets (not shown) are attached on the inner yokes 71 and 72. A frame fixing part 74 formed in a Π-shape is formed integrally on the bottom 76 of the yoke plate 70 to fix a frame 80 (FIGS. 6A and 6B). The magnets are formed on the opposing surfaces of the first and second inner yokes 71 and 72, facing a coil (not shown) to tilt a lens holder (not shown).
As shown in FIGS. 6A and 6B illustrating respectively the top and bottom views of the yoke plate 70 having the frame 80 therein, the frame 80 is formed of plastic material and inserted between left and right walls 75 of the yoke plate 70 and in the frame fixing part 74 through an insert forming process. Therefore, the frame fixing part 74 prevents the inserted frame 80 from moving. The frame fixing part 74 is formed at a portion of the yoke plate 70, which is integrally formed and bent from an end of the yoke plate 70 at a predetermined depth. The reference numerals 81 and 82 indicate a board assembling guide projection and a wire suspension passing hole, respectively.
As described above, since the frame 80 is formed on a rear portion of the yoke plate 70, it can tightly contact the left and right walls 75 and the bottom 76 of the yoke plate 70.
The second inner yokes 72 are formed by bending a portion of the bottom 76 of the yoke plate 70, such that a space 77 is formed on the yoke plate 70. Accordingly, since only a bottom portion of the second inner yokes 72 is secured to the bottom 76 of the yoke plate 70, the second yokes 72 are weak against vibration.
As the object lens 11 moves quickly during an operation of the optical pickup actuator, it creates vibration. This vibration, which is worsened by the weak disposition of the yokes 71 and 72 and by the integral formation of the yokes 71 and 72 of the yoke plate 70, is directly transmitted from the yokes 71 and 72 to the bottom and other parts of the yoke plate 70 and to the frame 80 (see arrows F1, F2 and F3 in FIGS. 7A and 7B). Further, since the frame 80 is tightly secured on the yoke plate 70 such that there is no gap between the frame 80 and the yoke plate 70, the vibration generated by the yoke plate 70 is fully transmitted to the frame 80. This interferes significantly with the effective and accurate operation of the actuator, thereby deteriorating the overall performance of the actuator greatly.